1. Field of the Invention
The present invention relates to a ground-fault resistance measurement circuit which measures a ground-fault resistance between a charge section insulated from a conductive frame body of electrical equipment and the frame body. It also relates to a ground-fault detection circuit provided with this.
2. Description of the Background Art
In recent years, hybrid cars have been widely popular which have both an engine and an electric motor. Along with this, electric cars, such as a fuel-battery vehicle, have also been put to more use. Such a vehicle which has an electric motor as its power source includes a high-voltage power source for driving the motor. In order to prevent a user from touching the vehicle's body and getting an electric shock, a charge section connected to the part of the high-voltage power source is insulated from the body. In this specification, the “charge section” means a “live part” defined in JIS B 9960-1 (the safety of machinery-a machine's electrical apparatus: general requirements). Specifically, it means a part which corresponds to “a conductor and a conductive part to which a voltage is applied in a normal usage state. They include a neutral conductor, but they do not usually include a PEN conductor (i.e., a protective conductor and an earth conductor which has the function of a neutral conductor)”.
In such a vehicle provided with a high-voltage part, a resistance value is measured between the charge section and the vehicle's body. If a ground fault is caused, the resistance drops between the charge section and the body. Then, a ground-fault detection circuit detects this drop in the resistance and notifies a user. A ground fault occurs, for example, when the covering of a cable is broken and it comes into contact with the vehicle body.
FIG. 6 is a circuit diagram, showing a schematic configuration of an electric circuit and a ground-fault resistance measurement circuit according to the background art used in such an electric automobile as described above, like a hybrid car or a fuel-battery vehicle (e.g., refer to Japanese Patent Laid-Open No. 2004-325381 specification). In each figure described below, component elements are identical with each other if given the same reference characters and numerals. Thus, their description is omitted.
A vehicle 101 shown in FIG. 6 includes: a motor 102; a secondary battery pack 103 which is formed by connecting a plurality of secondary batteries in series and outputs a high voltage of, for example, approximately 288 to 900 volts; and an inverter 104 which converts a direct-current voltage outputted from the secondary battery pack 103 into three-phase power-source voltages U, V, W for driving the motor 102. The secondary battery pack 103 is insulated from the vehicle 101's body, so that a user can be prevented from getting an electric shock.
In addition, the vehicle 101 includes a voltage monitoring circuit 105 which monitors an output voltage of the secondary battery pack 103, so as to control the charge and discharge of the secondary battery pack 103 and control the operation of the inverter 104 in response to the secondary battery pack 103's output voltage. To the voltage monitoring circuit 105, a power-source voltage for its operation is supplied by a secondary battery 106. This secondary battery 106 is provided separately from the secondary battery pack 103 and is formed by, for example, a 12-volt lead storage battery for a low-voltage system. The secondary battery 106 is connected at its negative electrode to a vehicle body 107, and the vehicle body 107 is set at the ground.
Thereby, the secondary battery pack 103 is connected via the internal resistances of the voltage monitoring circuit 105 to the vehicle body 107.
Furthermore, the vehicle 101 includes a ground-fault resistance measurement circuit which measures a resistance value between the charge section and the vehicle body so as to detect a drop in insulation resistance which may be caused by a ground fault or the like as described above. The ground-fault resistance measurement circuit is formed by a constant-current source 109 and a voltmeter 110. The constant-current source 109 feeds a constant direct current I from the positive electrode of the secondary battery pack 103 through a resistance 108 to the negative electrode of the secondary battery pack 103. The constant direct current I is set at a current value below a human-body sensible current, for example, 1 mA. If a ground fault is not produced, an electric current I1 which passes through the resistance 108 is almost equal to the direct current I. The voltage between both ends of the resistance 108 is measured by the voltmeter 110.
For example, if a ground fault causes the negative electrode of the secondary battery pack 103 to come into contact with the vehicle body 107, a resistance 111 generated by the ground fault is connected in parallel with the resistance 108. In FIG. 6, the resistance 108 is a resistance for measuring, using the voltmeter 110, a change in the electric current which is split by the resistance 11 as a change in the voltage. Thereby, the direct current I splits to the resistance 108 and the resistance 111, thus reducing the electric current I1 and then dropping the voltage measured by the voltmeter 110. This drop in the measured voltage by the voltmeter 110 helps detect a drop in insulation resistance which is caused by trouble such as a ground fault.
FIG. 7 is a circuit diagram, showing a schematic configuration of another electric circuit and a ground-fault resistance measurement circuit according to the background art used in such an electric automobile as described above, including a hybrid car and another vehicle (e.g., refer to Japanese Patent Laid-Open No. 8-70503 specification). In the same way as the vehicle 101 shown in FIG. 6, a vehicle 120 shown in FIG. 7 includes: a motor 102; a secondary battery pack 103; an inverter 104; a voltage monitoring circuit 105; a secondary battery 106; and a resistance 108. The secondary battery 106 is grounded via a vehicle body 107. Besides, the vehicle 120 includes aground-fault resistance measurement circuit 121 which measures a ground-fault resistance value Rx corresponding to the resistance value between an electrode of the secondary battery pack 103 and the vehicle body 107. In FIG. 7, the resistance 108 indicates a resistance which is produced by the internal resistances or the like of the voltage monitoring circuit 105.
The ground-fault resistance measurement circuit 121 is provided with a buffer 122 which supplies an alternating voltage Vx via a resistance 123 and a capacitor 124 to the negative electrode of the secondary battery pack 103, and a voltage measurement circuit 126 which measures, via a resistance 125, a voltage at the connection point of the capacitor 124 and the resistance 123. Then, the alternating voltage Vx outputted from the buffer 122 is supplied to the vehicle body 107, through the resistance 123, the capacitor 124 and the resistance 108.
Thereby, the alternating voltage Vx is divided by the series impedance consisting of the resistance 108 and the capacitor 124, and the resistance 123. Then, a division voltage value Vxb is measured by the voltage measurement circuit 126. Herein, if a ground fault takes place, the negative electrode of the secondary battery pack 103 is connected via a resistance 111 to the vehicle body 107. Thereby, the resistance 111 is connected in parallel with the resistance 108, so that a change is made in the ratio at which the alternating voltage Vx is divided. This also changes the division voltage value Vxb which is measured by the voltage measurement circuit 126. In other words, the division voltage value Vxb varies according to the ground-fault resistance value Rx which corresponds to the parallel resistance of the resistance 108 and the resistance 111. Therefore, based on the division voltage value Vxb, the ground-fault resistance value Rx can be measured. If the ground-fault resistance value Rx is below a predetermined threshold value, for example, 100 kΩ, then a drop in insulation resistance which is caused by trouble such as a ground fault is supposed to be detected.
As shown in FIG. 6, the direct current I is sent to the resistance 108 and the resistance 111, so that a ground fault is detected through the resistance 111. In such a configuration, in order to prevent a user from getting an electric shock when touching the vehicle body 107, the constant-current source 109 needs to be used in supplying the direct current I, so that the direct current I becomes a current value below a human-body sensible current. However, this presents a disadvantage in that a constant-current circuit like the constant-current source 109 makes the circuit configuration complicated. Besides, when the constant-current source 109 feeds the direct current I below a human-body sensible current, if a user touches the vehicle body 107, then the direct current I may continue flowing inside of the user's body for a long time without noticing it. This is undesirable.
In the ground-fault resistance measurement circuit 121 shown in FIG. 7, even if a ground fault is not generated, the ground-fault resistance value Rx is equivalent to the resistance 108's resistance value, for example, a resistance value of about 500 kΩ. In terms of a fuel-battery vehicle, water is generated in a fuel battery, so that the resistance 108's resistance value tends to drop. Thus, the ground-fault resistance value Rx becomes, for example, some 300 kΩ. On the other hand, if the output voltage of the secondary battery pack 103 is, for example, 600 volts, then in order to detect a ground fault which allows a human-body sensible current of 3 mA to flow, the occurrence of this ground fault needs detecting when the ground-fault resistance value Rx is below 200 kΩ. As described above, even at a normal time, the ground-fault resistance value Rx is approximately 300 kΩ. Therefore, for example, if the threshold-value voltage for detecting a ground fault is set at 250 kΩ, the ground-fault resistance value Rx needs to be measured with a precision of ±50 kΩ. Hence, there is a great demand for a higher measurement precision of the ground-fault resistance value Rx.
Herein, the division voltage value Vxb measured by the voltage measurement circuit 126 is obtained from the division voltage between the series impedance consisting of the ground-fault resistance value Rx and the capacitor 124, and the resistance 123. Thus, the lower the capacitor 124's impedance is. the higher the measurement precision of the ground-fault resistance value Rx which is measured based on the division voltage value Vxb becomes.
In order to lower the capacitor 124's impedance, heightening the frequency of the alternating voltage Vx, or increasing the capacitance of the capacitor 124, can be considered. However, in the vehicle 120 shown in FIG. 7, a switching noise made by a switching operation of the inverter 104 is inputted from the inverter 104 through the capacitor 124 and the resistance 125 to the voltage measurement circuit 126. Hence, in the voltage measurement circuit 126, the inverter 104's switching noise needs to be distinguished from the alternating voltage Vx's frequency used for measuring the ground-fault resistance value Rx. Thus, the alternating voltage Vx's frequency is set at a frequency far lower than the inverter 104's switching frequency, for example, 1 to 2 Hz. This makes it difficult to heighten the alternating voltage Vx's frequency.
In addition, if the alternating voltage Vx's frequency becomes higher, a leakage current flows via an opposite capacitance which is generated between the inverter 104 or the motor 102 and the vehicle body 107. This leakage current changes the division voltage value Vxb, thereby deteriorating the measurement precision of the ground-fault resistance value Rx measured based on the division voltage value Vxb. Accordingly, there is a disadvantage in that it is difficult to improve the measurement precision of the ground-fault resistance value Rx by heightening the alternating voltage Vx's frequency and lowering the capacitor 124's impedance.
Moreover, if the capacitor 124's capacitance becomes greater, the capacitor 124's discharge current increases. Thus, a larger quantity of electric current is sent, from the capacitor 124 through the ground-fault resistance value Rx and the vehicle body 107, to the human body touching the vehicle body 107. As a result, the capacitor 124's discharge is more likely to give an electric shock. Therefore, a disadvantage arises in that it is difficult to improve the measurement precision of the ground-fault resistance value Rx by increasing the capacitor 124's capacitance and lowering the capacitor 124's impedance.